Systems of this kind comprise a conveying structure for underwater production which makes it possible to bring the contents of an underwater reservoir or pipeline to a vessel to take it away, such as an oil tanker. All of the elements of this system must have structures that are resistant to icebergs and ice floes due to the layer of ice that covers certain areas periodically, or must be removed from the surface when the weather conditions at sea become too rough.
Usually, oil tankers are loaded at sea using a permanent anchorage consisting of a buoy anchored by chains or any combination of chains and cables. Loading is accomplished through a flexible pipe connecting the underwater reservoir, or the pipeline, to the buoy or directly to the ship being loaded. All of these loading stations are characterized by the permanent presence of an object located at the surface or in the immediately adjacent subsurface area (about 10 m beneath the surface of the ocean).
In oceans that are exposed to ice or to floes, such anchorages are destroyed immediately when they encounter an ice floe, or even when they are trapped in surface ice. Even if the buoy is designed to withstand the pressure of the ice locally, the total forces of interaction resulting from the drifting of ice floes at the anchorage are of such intensity that the mooring lines cannot resist it. It is therefore necessary to remove from the surface and from the subsurface all parts of the mooring system when there is a threat of ice or an encounter with an iceberg.
U.S. Pat. No. 4,650,431 describes a system allowing rapid disconnection of a ship being loaded from the transfer structure, with the pipes or tubes permitting the transfer of the output to the loading vessel being immersed at a level beneath the zone of turbulence or ice formation on the sea, yet at a certain height relative to the ocean bottom so that the flexible lines are not damaged by resting on the ocean bottom. This system has the disadvantage of leaving the flexible lines within the reach of icebergs, and the latter can have a considerable draft.
The document entitled "Sols for Floating Production Systems" issued during the conference entitled "Floating Production Systems" held in London on Dec. 11-12, 1989 describes a system which does not require any object at the surface to transfer production to a vessel, but which suffers from two serious disadvantages: it requires tankers with dynamic positioning and special equipment, said vessels having to be equipped with a specific system for loading, which requires these vessels to be dedicated to this operation. The multiplicity of loading systems on each vessel thus makes the economic feasibility of the system doubtful.